Alzheimer's disease affects approximately 20 to 40% of the population over 80 years of age, the fastest growing age group in the United States and other post-industrial countries. Common features in the brain of patients with Alzheimer's disease include the presence of abundant intraneuronal neurofibrillary tangles (NFTs) and extracellular amyloid rich β-amyloid plaques. NFTs are cytoskeletal pathologies largely composed of aggregates of hyperphosphorylated tau proteins assembled into periodically restricted amyloid fibers called paired helical filaments. The major component of amyloid plaques is a peptide, a small 39-43 aminoacid long β-amyloid peptide that is generated from the cleavage of a larger amyloid precursor protein. However, except for diffuse plaques formed almost exclusively of B-amyloid peptides, amyloid plaques are complex lesions containing numerous associated cellular products. Mutations causing increased production of the 42 amino acid form of this peptide have been genetically linked to autosomal dominant familial forms of Alzheimer's diseases. Deposits of β-amyloid occur very early in the disease process, long before clinical symptoms develop. Because these mutations appear to be pathogenic and cause Alzheimer's diseases in transgenic mice, β-amyloids are widely believed to play a causal role in the disease. Whether or not amyloid deposits are causal, they are certainly a key part of the diagnosis. Further, because amyloid plaques occur early in the disease, the ability to image deposits would provide a convenient marker for early diagnosis and prevention of the disease as well as a method for monitoring the effectiveness of therapeutic regimens.
Alzheimer's disease is currently definitively diagnosed by taking sections from postmortem brain and quantifying the density of neocortical amyloid deposits. Unfortunately, current techniques for detecting amyloid deposits and/or NFTs require postmortem or biopsy analysis. For example, thioflavin fluorescent-labeling of amyloid in brain sections in vitro is currently a widely-used method for evaluation of the brain. Another potential amyloid probe, Chrysamine-G, a congo red derivative, has also been developed. Congo red is a charged molecule and thus lacks sufficient hydrophobicity for diffusion through the blood brain barrier and is therefore not useful as an in vivo label. See Klunk et al, Neurobiology of Aging, 16:541-548 (1995), and PCT Publication No. WO 96/34853. Chrysamine G enters the blood brain barrier better than Congo red, but its ability to label amyloid plaques in Alzheimer's brain appears weak. See for example, H. Han, C-G Cho and P. T. Lansbury, Jr J. Am. Chem. Soc. 118, 4506 (1996); N. A. Dezutter et al, J. Label. Compd. Radiopharm. 42, 309 (1999). Similarly, earlier attempts to use monoclonal antibodies as probes for in-vivo imaging of β-amyloid were hampered by their limited ability to cross the blood brain barrier. See R. E. Majocha et al, J. Nucl. Med. 33, 2184 (1992). More recently, the use of monobiotinylated conjugates of 1251-Aβ 1-40 with permeability through the blood brain barrier has also been proposed (See Y. Saito et al., Proc. Natl. Acad. Sci. USA 22, 2288 (1991)), but its ability to label β-amyloid plaques and/or NFTs in vivo'has not yet been demonstrated. Quantitation of the deposits in vivo is not yet possible with the currently available probes. Accordingly, a need exists for a convenient marker for early diagnosis of Alzheimer's disease.
In vivo, non invasive determination of regional cerebral glucose metabolic rates (rCMRGl) with positron emission tomography (PET) has been an important tool in the assessment of brain function in Alzheimer's disease patients. Numerous studies using 2-[F-18]fluoro-2-deoxy-D-glucose (FDG) have demonstrated a characteristic metabolic pattern of hypometabolism in temporoparietal and frontal association areas. A few of these studies have compared rCMRGl with postmortem regional neuronal pathology. These results and the uncertainties of the Alzheimer's disease pathogenic cascade highlight the importance of assessing amyloid and neurofibril deposition in vivo, non-invasively in these patients.